Turbine bucket shroud arrangement and method of controlling turbine bucket interaction with an adjacent turbine bucket

ABSTRACT

A turbine bucket shroud arrangement for a turbine system includes a contact region of a tip shroud, wherein the contact region is in close proximity to an adjacent tip shroud. Also included is a negative thermal expansion material disposed proximate the contact region, the contact region comprising a first volume during a startup condition and a shutdown condition of the turbine system and a second volume during a steady state condition of the turbine system, wherein the second volume is less than the first volume.

BACKGROUND OF THE INVENTION

The subject matter disclosed herein relates to turbine systems, and moreparticularly to turbine bucket shroud arrangements, as well as a methodof controlling turbine bucket interaction with an adjacent turbinebucket.

Turbine systems employ a number of rotating components or assemblies,such as compressor stages and turbine stages that rotate at high speedwhen the turbine is in operation, for example. In general, a stageincludes a plurality of free-floating blades that extend radiallyoutward from a central hub. Some blades include a shroud that limitsvibration within a stage and provides sealing to increase efficiency ofthe overall system. The shroud is typically positioned at a tip portionof the blade, a mid-portion of the blade or at both the mid portion andthe tip portion of the blade. The shrouds are designed such that thefree-floating blades interlock to form an integral rotating memberduring operation.

Prior to rotation of the free-floating blades, a gap between contactsurfaces of the shrouds is present. The distance of the gap determineshow early an interlock of the shrouds occurs upon startup of the turbinesystem. Too large of a gap inefficiently delays the locking speed, whichmay result in resonance, for example. Too small of a gap results inundesirable effects at high speed operation of the turbine system. Sucheffects include lower damping effectiveness and flutter margin, as wellas high stresses imposed on the turbine bucket due to increased transferof forces between the contacting shrouds, for example. Therefore,current efforts to beneficially reduce the gap to provide an earlyinterlock to address potential low speed aeromechanics issues aremitigated by the detrimental effects on tip shroud life that occur atsteady state operating conditions.

BRIEF DESCRIPTION OF THE INVENTION

According to one aspect of the invention, a turbine bucket shroudarrangement for a turbine system includes a contact region of a tipshroud, wherein the contact region is in close proximity to an adjacenttip shroud. Also included is a negative thermal expansion materialdisposed proximate the contact region, the contact region comprising afirst volume during a startup condition and a shutdown condition of theturbine system and a second volume during a steady state condition ofthe turbine system, wherein the second volume is less than the firstvolume.

According to another aspect of the invention, a method of controllingturbine bucket interaction with an adjacent turbine bucket is provided.The method includes reducing a gap disposed between a contact region ofa tip shroud and an adjacent tip shroud by depositing a negative thermalexpansion material proximate the contact region. Also included isengaging the contact region of the tip shroud with the adjacent tipshroud during a startup operating condition and a shutdown operatingcondition. Further included is decreasing a volume of the contact regionduring increased temperature operating conditions upon contraction ofthe negative thermal expansion material, wherein decreasing the volumereduces tip shroud contact forces and stresses during a steady stateoperating condition.

These and other advantages and features will become more apparent fromthe following description taken in conjunction with the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The subject matter, which is regarded as the invention, is particularlypointed out and distinctly claimed in the claims at the conclusion ofthe specification. The foregoing and other features, and advantages ofthe invention are apparent from the following detailed description takenin conjunction with the accompanying drawings in which:

FIG. 1 is a schematic view of a turbine system;

FIG. 2 is a partial perspective view of a turbine stage of the turbinesystem;

FIG. 3 is a top plan view of a turbine bucket shroud arrangement havinga contact region;

FIG. 4 is an enlarged top plan view of the contact region of FIG. 3;

FIG. 5 is a schematic view of the contact region comprising acomposition;

FIG. 6 is a schematic view of a plurality of layers of the composition;and

FIG. 7 is a flow diagram illustrating a method of controlling turbinebucket interaction with an adjacent turbine bucket.

The detailed description explains embodiments of the invention, togetherwith advantages and features, by way of example with reference to thedrawings.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 1, a turbine system, shown in the form of a gasturbine engine, constructed in accordance with an exemplary embodimentof the present invention, is indicated generally at 10. The turbinesystem 10 includes a compressor 12 and a plurality of combustorassemblies arranged in a can annular array, one of which is indicated at14. As shown, the combustor assembly 14 includes an endcover assembly 16that seals, and at least partially defines, a combustion chamber 18. Aplurality of nozzles 20-22 are supported by the endcover assembly 16 andextend into the combustion chamber 18. The nozzles 20-22 receive fuelthrough a common fuel inlet (not shown) and compressed air from thecompressor 12. The fuel and compressed air are passed into thecombustion chamber 18 and ignited to form a high temperature, highpressure combustion product or air stream that is used to drive aturbine 24. The turbine 24 includes a plurality of stages 26-28 that areoperationally connected to the compressor 12 through acompressor/turbine shaft 30 (also referred to as a rotor).

In operation, air flows into the compressor 12 and is compressed into ahigh pressure gas. The high pressure gas is supplied to the combustorassembly 14 and mixed with fuel, for example process gas and/orsynthetic gas (syngas), in the combustion chamber 18. The fuel/air orcombustible mixture ignites to form a high pressure, high temperaturecombustion gas stream. Alternatively, the combustor assembly 14 cancombust fuels that include, but are not limited to, natural gas and/orfuel oil. In any event, the combustor assembly 14 channels thecombustion gas stream to the turbine 24 which converts thermal energy tomechanical, rotational energy.

At this point, it should be understood that each of the plurality ofstages 26-28 is similarly formed, thus reference will be made to FIG. 2in describing stage 26 constructed in accordance with an exemplaryembodiment of the present invention with an understanding that theremaining stages, i.e., stages 27 and 28, have corresponding structure.Also, it should be understood that the present invention could beemployed in stages in the compressor 12 or other rotating assembliesthat require wear and/or impact resistant surfaces. In any event, thestage 26 is shown to include a plurality of rotating members, such as anairfoil 32, which each extend radially outward from a central hub 34having an axial centerline 35. The airfoil 32 is rotatable about theaxial centerline 35 of the central hub 34 and includes a base portion 36and a tip portion 38.

A tip shroud 50 covers the tip portion 38 of the airfoil 32. The tipshroud 50 is designed to receive, or nest with, tip shrouds on adjacentrotating members in order to form a continuous ring that extendscircumferentially about the stage 26. The continuous ring creates anouter flow path boundary that reduces gas path air leakage over topportions (not separately labeled) of the stage 26, so as to increasestage efficiency and overall turbine performance. In the exemplaryembodiment shown, during high or operational speeds, adjacent airfoilsinterlock through the tip shroud 50 of each respective airfoil by virtueof centrifugal forces and thermal loads created by the operation of theturbine 24.

Referring now to FIGS. 3 and 4, the tip shroud 50 is illustrated ingreater detail and is in close proximity with an adjacent tip shroud 52.The tip shroud 50 includes a contact region 54 configured to engage theadjacent tip shroud 52 during operation of the turbine system 10.Specifically, the contact region 54 engages an adjacent contact region56 of the adjacent tip shroud 52. A gap 58 is present between the tipshroud 50 and the adjacent tip shroud 52, and more particularly betweenthe contact region 54 and the adjacent contact region 56. The gap 58 ispresent prior to startup of the turbine system 10. The gap 58 isdimensionally selected based on a desirable early interlock of the tipshroud 50 and the adjacent tip shroud 52 upon operation of the turbinesystem 10 and rotation of the airfoil 32. Subsequent to interlock of thetip shroud 50 and the adjacent tip shroud 52, the operating environmentincreases in temperature, thereby resulting in thermal expansion of mostcomponents within the turbine 24.

To alleviate the stresses imposed by potential expansion of alreadycontacted components, at least one of the contact region 54 and theadjacent contact region 56, but typically both the contact region 54 andthe adjacent contact region 56, include a negative thermal expansionmaterial 60. The negative thermal expansion material 60 is defined byhaving a negative coefficient of thermal expansion, such that thematerial contracts in response to increased temperature exposure, ratherthan expanding. It is to be appreciated that any material having anegative coefficient of thermal expansion may be suitable for inclusionwith the contact region 54 and the adjacent contact region 56. Examplesof such materials include zircon, zirconium tungstate and A₂(MO₄)₃compounds. Forming at least a portion of the contact region 54 and theadjacent contact region 56 with the negative thermal expansion material60 advantageously allows for the gap 58 to be dimensionally reduced tofacilitate an early interlock between the tip shroud 50 and the adjacenttip shroud 52, while also reducing the contact forces associated withinteraction between the tip shroud 50 and the adjacent tip shroud 52,thereby reducing stresses imposed on various portions of the tip shroud50, the adjacent tip shroud 52 and the airfoil 32 attached thereto. Thestress reduction is achieved by maintaining an interlock, butcontracting the negative thermal expansion material 60. In other words,the contact region 54 comprises a first volume during a startupcondition of the turbine system 10 and a smaller, second volume during asteady state operating condition of the turbine system 10.

Referring now to FIGS. 5 and 6, the contact region 54 is schematicallyillustrated in greater detail. The tip shroud 50 includes a base metalregion 62 that is coated or integrally formed with the contact region54. The contact region 54 is formed of one or more composition layersthat typically include a fraction of the negative thermal expansionmaterial 60 and a fraction of a wear resistant material. As noted above,the contact region 54 may include a single composition layer (FIG. 5) ora plurality of composition layers (FIG. 6). In an embodiment having aplurality of composition layers 72, it is to be appreciated thatdistinct volume and/or weight fractions of the negative thermalexpansion material 60 may be present in the plurality of compositionlayers 72, such as a first layer 64, a second layer 68 and a third layer70, as shown. In one embodiment, the fraction of the negative thermalexpansion material 60 progressively increases in each layer, relative tomoving away from the base metal region 62. Specifically, the first layer64 may include a lower fraction of the negative thermal expansionmaterial 60 than the second layer 68, with the second layer 68 having alower fraction than the third layer 70. Gradually transitioning theinclusion of the negative thermal expansion material 60 from the basemetal region 62 reduces thermal fight at the interface between thecontact region 54 and the base metal region 62 of the tip shroud 50. Itis to be appreciated that each of the plurality of composition layers 72may vary in thickness from one another and may comprise the negativethermal expansion material 60 in a fraction ranging from about 0% toabout 100%.

The contact region 54, whether a single layer or the plurality ofcomposition layers 72, may be deposited or integrated with the tipshroud 50 in a number of application processes. Examples of suchprocesses include brazing, welding, laser cladding, cold spraying and aplasma transferred arc (PTA) process. The preceding list is merelyillustrative and is not intended to be limiting of numerous othersuitable application procedures.

As illustrated in the flow diagram of FIG. 7, and with reference toFIGS. 1-6, a method of controlling turbine bucket interaction with anadjacent turbine bucket 100 is also provided. The turbine system 10, aswell as the tip shroud 50 and the contact region 54, have beenpreviously described and specific structural components need not bedescribed in further detail. The method of controlling turbine bucketinteraction with an adjacent turbine bucket 100 includes reducing a gapbetween a contact region of a tip shroud and an adjacent tip shroud bydepositing a negative thermal expansion material proximate the contactregion 102. The contact region is engaged with the adjacent tip shroudduring a startup operating condition 104. A volume of the contact regionis decreased during increased temperature operating conditions uponcontraction of the negative thermal expansion material 106.

While the invention has been described in detail in connection with onlya limited number of embodiments, it should be readily understood thatthe invention is not limited to such disclosed embodiments. Rather, theinvention can be modified to incorporate any number of variations,alterations, substitutions or equivalent arrangements not heretoforedescribed, but which are commensurate with the spirit and scope of theinvention. Additionally, while various embodiments of the invention havebeen described, it is to be understood that aspects of the invention mayinclude only some of the described embodiments. Accordingly, theinvention is not to be seen as limited by the foregoing description, butis only limited by the scope of the appended claims.

1. A turbine bucket shroud arrangement for a turbine system comprising:a contact region of a tip shroud, wherein the contact region is in closeproximity to an adjacent tip shroud; and a negative thermal expansionmaterial disposed proximate the contact region, the contact regioncomprising a first volume during a startup condition and a shutdowncondition of the turbine system and a second volume during a steadystate condition of the turbine system, wherein the second volume is lessthan the first volume.
 2. The turbine bucket shroud arrangement of claim1, wherein the negative thermal expansion material comprises at leastone of zircon, zirconium tungstate and an A₂(MO₄)₃ compound.
 3. Theturbine bucket shroud arrangement of claim 1, further comprising anadjacent contact region of the adjacent tip shroud.
 4. The turbinebucket shroud arrangement of claim 3, wherein the adjacent contactregion comprises a negative thermal expansion material.
 5. The turbinebucket shroud arrangement of claim 1, wherein the contact regioncomprises a composition of the negative thermal expansion material and awear resistant material.
 6. The turbine bucket shroud arrangement ofclaim 5, wherein the composition comprises a layer disposed on a basemetal of the tip shroud.
 7. The turbine bucket shroud arrangement ofclaim 5, wherein the composition comprises a plurality of layersdisposed on a base metal of the tip shroud.
 8. The turbine bucket shroudarrangement of claim 7, wherein each of the plurality of layers comprisea distinct volume fraction of the negative thermal expansion material.9. The turbine bucket shroud arrangement of claim 5, wherein thecomposition is brazed to the tip shroud.
 10. The turbine bucket shroudarrangement of claim 5, wherein the composition is welded to the tipshroud.
 11. A method of controlling turbine bucket interaction with anadjacent turbine bucket comprising: reducing a gap disposed between acontact region of a tip shroud and an adjacent tip shroud by depositinga negative thermal expansion material proximate the contact region;engaging the contact region of the tip shroud with the adjacent tipshroud during a startup operating condition and a shutdown operatingcondition; and decreasing a volume of the contact region duringincreased temperature operating conditions upon contraction of thenegative thermal expansion material, wherein decreasing the volumereduces turbine bucket tip shroud contact forces and stresses during asteady state operating condition.
 12. The method of claim 11, furthercomprising depositing the negative thermal expansion material proximatean adjacent contact region of the adjacent tip shroud.
 13. The method ofclaim 11, further comprising forming a composition proximate the contactregion, wherein the composition comprises the negative thermal expansionmaterial and a wear resistant material.
 14. The method of claim 13,further comprising forming a plurality of layers of the compositionproximate the contact region.
 15. The method of claim 14, wherein eachof the plurality of layers comprises a distinct volume fraction of thenegative thermal expansion material.
 16. The method of claim 13, whereinthe composition is brazed to the tip shroud.
 17. The method of claim 13,wherein the composition is welded to the tip shroud.
 18. The method ofclaim 13, wherein the composition is laser cladded to the tip shroud.19. The method of claim 13, wherein the composition is cold sprayed ontothe tip shroud.
 20. The method of claim 13, wherein the composition isdeposited during a plasma transferred arc process.